National Geographic : 1992 May
To MAKE A DNA PRINT Deep in the animated network of the cell nucleus lies the mole cule of heredity-DNA. Its twin spirals are built from four interlocking chemical bases adenine paired with thymine and cytosine paired with gua nine. Code messages, genes, are stored along a chromosome in sequences of these chemical bases. Genes define the unique characteristics of each living thing. It is here, for example, that butterflies are given their wing patterns and people are assigned eye color. Scientists have been chal lenged by the inner workings of DNA since 1944, when Ameri can researcher Oswald Avery defined its role in transferring he reditary characteris tics. James D. Watson and Francis Crick described the spiral structure of DNA in B 1953. In 1984 Alec Jeffreys, a geneti cist in Leicester, England, devised a way to visually iden tify DNA found be tween the genes. In certain regions the DNA patterns vary distinctively from person to person except in cases of identical twins. Jeffreys's method of identification, known as DNA fingerprinting (simpli fied in the painting at right), has become a valuable technique for investigating crimes in which biological clues are left behind. Now, in some jurisdictions, sus pects can be linked to a crime by evidence that incriminates them to their very molecules. *4 ,9 TARGETED DNA SEQUENCE \ CYTOSINE- GUANINE---. . ' THYMINE -ADENINE 3CUT DNA INTO FRAGMENTS Enzymes that recognize certain sequences in the chemical base patterns are added to the DNA. These enzymes, proteins that cause a chemical reaction, act like molecular scissors and cut the DNA molecule at specific points, leaving frag ments of various lengths. 2EXTRACT AND PURIFY DNA The sample is treated with chemicals to break open the white blood cells. Ina centrifuge, DNA is separated from the cells and later purified. 1W COLLECT THE SAMPLE The bloodied shirt of a mur der victim contains enough of the wounded killer's white blood cells to draw a sample. DNA can also be extracted from traces of semen, saliva, hair roots, or bone-wherever nucleated cells are found. 116 P900(0 o~-9 p9 o do\"